Attempts have been made to produce copolymers of styrene and isobutylene. In general, the polymerization reactions have been carried out in the presence of Friedel-Crafts catalysts in solvents such as alkyl halides or saturated hydrocarbons such as propane, heptane, methylchloride, or ethylene dichloride.
The prior art products, for the most part, have not, in fact, been copolymers. They have for the large part been mixtures of homopolymers of styrene and isobutylene and contain some copolymers. This is substantiated by the fact that solutions of the so-called styrene-isobutylene copolymers in hexane or toluene show stratification or phase separation. These stratified layers indicate isotropic properties within each layer; furthermore, pads or films made with prior art copolymers tend to be translucent or opaque. A lack of transparency is a result of the fact that the various homopolymers, i.e., polystyrene and of isobutylene are not mutually soluble within one another and therefore form sites of precipitated material which result in opaqueness or translucency. Furthermore, a determination of the glass transition by differential thermal analysis demonstrates that the products have at least two broad glass transition temperatures. A homogeneous copolymer would have a single glass transition point whereas the mixed homopolymer blends show more than one glass transition temperature of a broad range.
Some improvement in the homogenity of the polymer produced is disclosed in U.S. Pat. Nos. 2,641,595 and 2,730,519, incorporated herein by reference. These improvements were effected by recognizing the importance of conducting the polymerization continuously under steady-state conditions in a well mixed reactor. However, while the copolymers of these patents were vastly more homogeneous than the prior art compositions, they still showed a fairly broad composition range because: (1) the importance of balancing reactivity ratios of the two monomers was not recognized; (2) the reactor mixing requirements to insure sufficient approach to an ideal back-mixed reactor were not recognized and specified; and (3) the necessity for maintaining polymer properly in solution was not recognized. Using the teachings of these patents, it was not possible to produce even reasonably good copolymers at low styrene content and it was not possible to operate at high conversion which is economically desirable because of the severe problems associated with recovery and recycle of unreacted styrene. Furthermore, even under the most ideal conditions for homogenity of high styrene content (&gt;58%) and low conversion (&lt;80%), the copolymers produced by the process of the aforementioned methods were far less homogeneous than those which can be produced by the present invention.